Olive kernel can play an important role as a fuel for heat and power production in the island of Crete, substituting a large part of conventional fuels. However, combustion of this biofuel may create operational and environmental problems related to its inorganic constituents. Thus, the thermal behaviour of the ashes in terms of slagging and fouling propensities and their environmental impact upon disposal to local soils were investigated, through lab-scale fixed/fluidized bed combustion tests. Bottom and fly ashes were characterized by mineralogical, chemical, morphological and fusibility analyses, as well as standard leaching tests and the results under the different combustion configurations were compared. Olive kernel ash was rich in Ca, Si and P minerals and contained substantial amounts of alkali. Under the conditions of the combustion tests, no signs of bed agglomeration or ash deposition were noticed; however, when combustion takes place in a fixed bed this should be operated below 1100 degrees C, to avoid ash melting and the companion problems. Trace elements showed little preference for the fly ash. The elements Cr, Cu, Ni and Mn were enriched in fixed bed ash. Toxic metal ions were released in low quantities in the soil, below the legislative limit values, with the exception of Cr. The low leachability of trace elements from the fixed bed ash was attributed to the alkaline nature of the ashes, the mineralogy, the chemistry and the buffering capacity of the soil. The high extraction rates of Mn, Zn and Cr, from the fly ash, suggest that these elements were associated with carbonates, sulfides, sulfates or organic matter. (c) 2008 Elsevier Ltd. All rights reserved.